In microelectronic applications, polymers that demonstrate high temperature resistance are generally well known. Precursors of such polymers, such as polyimides and polybenzoxazoles can be made photoreactive with suitable additives. The precursors are converted to the desired polymer by known techniques such as exposure to high temperatures. The polymer precursors are used to prepare protective layers, insulating layers, and relief structures of highly heat-resistant polymers.
Conventional positive-working photosensitive polybenzoxazoles (PBO) contain an alkaline soluble PBO precursor and a diazoquinone photoactive compound as shown in U.S. Pat. No. 4,371,685. The diazoquinone compound inhibits the solubility of the PBO precursor in an aqueous base. After exposure to light, the diazoquinone compound undergoes photolysis and converts to indenecarboxylic acid, which promotes the aqueous base solubility of the PBO precursor.
Applications for photosensitive polybenzoxazole precursor resin compositions typically require film thicknesses in excess of 5 microns. As a result, the absorbance by the photosensitive composition are typically very high, which results in higher exposure requirements (resulting in slower throughput in manufacturing) and a degradation of image acuity. The photoactive agents are one of the primary species raising the absorbance of the photosensitive polybenzoxazole precursor resin compositions. Lowering the absorbance by decreasing the amount of photoactive agents, typically results in lowering the image acuity and increasing the amount of unexposed film thickness loss. We have surprisingly found that positive photosensitive formulations prepared according to the present invention compared to the prior art decrease the optical absorbance of films prepared from the formulations without substantially affecting the unexposed film dissolution. Consequently, the positive photosensitive formulations prepared according to the present invention have improved photospeed when exposed in i-line and g-line radiation while maintaining high contrast.